*{
    padding: 0;
    margin: 0;
}
.box{
    height: 500px;
    width: 500px;
    margin: 10px auto;
    background-color: yellow;
}
.box div{
    box-sizing: border-box;
    width: 10px;
    height: 10px;
    float: left;
    border: 1px solid #424242;
}